Crinoids have an external skeleton made of calcium carbonate plates covered by a thin skin. The plates are held together with ligaments or muscles. Crinoids are the oldest of the living echinoderms with a fossil record stretching back 450 million years. Most Crinoids were filter feeders consuming plankton and decaying organic matter. To feed they spread their feeding arms to sieve the passing sea water for microscopic organisms. Living relatives include Feather stars and Sea lilies.